Behind-stop tessar type photographic lens system

ABSTRACT

A behind-stop Tessar type photographic lens system having a sufficiently long back focal length for which cutting of marginal rays by the stop is prevented and aberrations are corrected favourably.

ITI O'I\ United Sta- 40 7 D Imai BEHIND-STOP TESSAR TYPE PHOTOGRAPHIC LENS SYSTEM Toshihiro lmai, Tokyo, Japan Olympus Optical Co., Ltd., Tokyo, Japan Filed: June 24, 1974 Appl. No.: 482,762

Inventor:

'Assignee:

Foreign Application Priority Data June 26, 1973 Japan 48-72410 US. Cl 350/227; 350/206 Int. Cl. G02b 9/20 Field of Search 350/227 [451 July '22, 1975 [56] References Cited UNITED STATES PATENTS 3,615,126 10/1971 Kojima 350/227 Primary Examiner-John K. Corbin Attorney, Agent, or Firm-Cushman, Darby & Cushman [57] ABSTRACT A behind-stop Tessar type photographic lens system having a sufficiently long back focal length for which cutting of marginal rays by the stop is prevented and aberrations are corrected favourably.

1 Claim, 4 Drawing Figures PATENTEI] JUL 2 2 ms FIG. 20

FIG. 28

FIG. 2A

SPHERICAL ABERRATION ASTIGMATISM DISTORTION BEHIND-STOP TESSAR TYPE PHOTOGRAPHIC LENS SYSTEM BACKGROUND OF THE INVENTION a. Field of the Invention The present invention relates to a behind-stop Tessar SUMMARY OF THE INVENTION It is, therefore, a primary object of the present invention to provide a behind-stop Tessar type photographic lens system for which aberrations for offaxial ray are corrected favourably, said behind-stop Tessar type photographic lens system not causing cutting of rays by the stop over a large field angle and having an adequate back focal length.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a sectional view of the lens system according to the present invention; and

FIGS. 2A, 2B and 2C respectively show graphs illustrating aberration curves of the lens system according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The concrete contents of the lens system according to the present invention are described below. FIG. 1 shows a sectional view of the behind-stop Tessar type photographic lens system according to the present invention. As shown in said figure, the lens system according to the present invention comprises three lens components of four lenses in which the first lens component nearest the object is a positive meniscus lens with its convex surface positioned on the object side, a second lens component is a biconcave lens and a third lens component is a cemented biconvex doublet lens comprising a negative meniscus lens and a biconvex lens, the lens system according to the present invention further having numerical data as given below.

f= l,f,, 0.800, 2w= 60. 1:2.8

In the above, reference symbols r, through r respectively represent radii of curvature of respective surfaces of lenses, reference symbols d, through d respectively represent thicknesses of respective lenses and airspaces between lenses, reference symbols m through m respectively represent refractive indexes for d-line of respective lenses, and reference symbols v, through 1 respectively represent Abbes numbers of respective lenses.

For the lens system according to the present invention having the above numerical data, cutting of marginal rays by the stop is prevented by making thicknesses of respective lenses and airspaces between respective lenses as small as possible for a lens system of F/2.8. Besides, by making the radius of curvature r of the surface on the object side of the second lens component as 1.6f r -l.2f, aberration toward the plus side for upper rays at the zonal portion is prevented from occurring, and favourable symmetry of coma is attained for offaxial rays at the zonal portion. Further, by making the radius of curvature r, of the front surface of the lens system as 0.3f r 0.4f, insufficient correction of upper rays at the zonal portion and insufficient correction of spherical aberration are prevented.

Moreover, to prevent increase of Petzvals sum, the refractive index n. of the first lens component and refractive index n., of the lens on the image side of the third lens component, both lenses being convex lenses, are made to have large values and, at the same time, are limited to a value within 1.8 in order to avoid using special materials. That is, for said lenses, refractive indexes are selected as values within ranges of 1.7 r1 1.8 and 1.7 n., 1.8.

Besides, in order to favourably correct chromatic aberration, materials of both lenses of the third lens component are selected from low-dispersion glasses of 11 11 30.

Aberration curves of the lens system according to the present invention are shown in FIGS. 2A, 2B and 2C. Out of them, FIG. 2A shows spherical aberration, FIG. 2B shows astigmatism and FIG. 2C shows distortion. Seidels coefficients of aberrations in case of f 1.0 are given in the table below. In said table, reference symbol SI represents the coefficient of spherical aberration, reference symbol 811 represents the coefficient of coma, reference symbol SIII represents the coefficient of astigmatism, reference symbol P represents Petzvals sum and reference symbol SV represents the coefficient of distortion.

aberrations, the lens system according to the present invention is a behind-stop Tessar type lens system which does not cause cutting of marginal rays by the stop and for which aberrations are corrected quite 111F001) "2 7270 V2 3210 favourably. n 3 4 I claim: 4 0-037 l. A behind-stop Tessar type photographic lens sys- L617 d, =0 023 n" L607 :4034 tem comprises three lens components of four lenses, a 5 rh- =028| i first lens component being a positive meniscus lens "4:175700 with its convex surface positioned on the object side. a second lens component being a biconcave lens and a third lens component being a cemented doublet lens i h above, r f n e b l r1 h h r7 respec- Comprising a negative meniscus lens and i1 biCOnVeX l0 tively represent radii ofcurvature of respective surlens, Said behind-Stop Tessa! yp Photographic lens faces of lenses, reference symbols d, through d re System having the following numerical dam: spectively represent thicknesses of respective lenses and airspaces between lenses, reference symbols n through n respectively represent rel fractive indexes for d-lme of respective lenses, and r. 0.348 7 reference symbols 11 through 11 respectively reprer 0 888 4:01) "78590 sent Abbes numbers of respective lenses. 

1. A behind-stop Tessar type photographic lens system comprises three lens components of four lenses, a first lens component being a positive meniscus lens with its convex surface positioned on the object side, a second lens component being a biconcave lens and a third lens component being a cemented doublet lens comprising a negative meniscus lens and a biconvex lens, said behind-stop Tessar type photographic lens system having the following numerical data: 